GENERAL-PURPOSE TIMER
M68HC16 Z SERIES
11-10
USER’S MANUAL
The prescaler can be read at any time. In freeze mode the prescaler can also be writ-
ten. Word accesses must be used to ensure coherency. If coherency is not needed
byte accesses can be used. The prescaler value is contained in bits [8:0] while bits
[15:9] are unimplemented and are read as zeros.
Multiplexer outputs (including the PCLK signal) can be connected to external pins. The
CPROUT bit in the TMSK2 register configures the OC1pin to output the TCNT clock
and the PPROUT bit in the PWMC register configures the PWMA pin to output the
PWMC clock. CPROUT and PPROUT can be written at any time. Clock signals on
OC1 and PWMA do not have a 50% duty cycle. They have the period of the selected
clock but are high for only one system clock time.
The prescaler also supplies three clock signals to the pulse accumulator clock select
mux. These are the system clock divided by 512, the external clock signal from the
PCLK pin and the capture/compare clock signal.
11.8 Capture/Compare Unit
The capture/compare unit contains the timer counter (TCNT), the input capture (IC)
functions and the output compare (OC) functions.
is a block diagram of
the capture/compare unit.
11.8.1 Timer Counter
The timer counter (TCNT) is the key timing component in the capture/compare unit.
The timer counter is a 16-bit free-running counter that starts counting after the proces-
sor comes out of reset. The counter cannot be stopped during normal operation. After
reset, the GPT is configured to use the system clock divided by four as the input to the
counter. The prescaler divides the system clock and provides selectable input fre-
quencies. User software can configure the system to use one of seven prescaler out-
puts or an external clock.
The counter can be read any time without affecting its value. Because the GPT is in-
terfaced to the IMB, and the IMB supports a 16-bit bus, a word read gives a coherent
value. If coherency is not needed, byte accesses can be made. The counter is set to
$0000 during reset and is normally a read-only register. In test mode and freeze mode,
any value can be written to the timer counter.
When the counter rolls over from $FFFF to $0000, the timer overflow flag (TOF) in tim-
er interrupt flag register 2 (TFLG2) is set. An interrupt can be enabled by setting the
corresponding interrupt enable bit (TOI) in timer interrupt mask register 2 (TMSK2).
Refer to
11.8.2 Input Capture Functions
All GPT input capture functions use the same 16-bit timer counter (TCNT). Each input
capture pin has a dedicated 16-bit latch and input edge-detection/selection logic. Each
input capture function has an associated status flag, and can cause the GPT to make
an interrupt service request.
When a selected edge transition occurs on an input capture pin, the associated 16-bit
latch captures the content of TCNT and sets the appropriate status flag. An interrupt
request can be generated when the transition is detected.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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